2021 EJC Prelims Paper 2 (solution with comments)

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2021 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 0 – REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions 9729/02 16 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not use paper clips, highlighters, glue or correction fluid. Write your name, civics group, registration number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Answer all questions on the Question Paper The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. For Examiner’s Use Paper 2 1 / 15 2 / 12 3 / 13 4 / 15 5 / 20 Total / 75 This document consists of 19 printed pages and 1 blank page.

2 © EJC 9729/02/J2PE/21 For Examiner’s Use 1 Nitrosyl chloride, NOCl decomposes at temperatures above 100 oC to form nitrogen monoxide and chlorine as shown in the equation below. 2NOCl(g) → 2NO(g) + Cl2(g) (a) A sample of NOCl was allowed to decompose at 127 ºC and 147 atm. Assuming that NOCl behaves ideally, show that its initial concentration is 4.48 mol dm−3. [2] Marker’s Comments: • Generally well done, but a number of students were not clear in showing the conversion of m −3 to dm −3. Some students thought that volume was calculated in cm −3 instead of m−3. (b) A student pointed out that the assumption in (a) that NOCl behaves ideally is not valid. Suggest and explain two possible reasons why this may be so. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ .............................. [2] Marker’s Comments: • A significant number of students stated that NOCl was a large molecule, which led to its volume becoming significant. This is not accepted as NOCl only comprises 3 atoms and is not large enough for the assertion to hold. • A number of students were still making the same mistake of stating that the volume o f the gas (instead of gas particle) is significant compared to the volume of the container, without realising that the volume of the gas is the volume of the container. • Some students tried to argue in terms of the temperature being too high/low, but 127 ºC is neither too high or too low to attribute any deviation from ideality to.   ( ) 34.48 mol dm pV nRT np V RT − − = == = + = = = 3 3 NOC 147 101325 8.31 127 273 4480 mol m 4480 mol dm1000 − l The pressure of NOCl is very high , causing the volume of each gas particle becomes more significant relative to the total volume of the gas. NOCl has significant intermolecular forces of attraction as it is polar/has a relatively large polarizable electron cloud.

3 © EJC 9729/02/J2PE/21 [Turn Over For Examiner’s Use (c) The change in the concentration of NOC l was monitored as the decomposition reaction occurred over time. The data obtained is shown in Fig. 1.1. Fig. 1.1 (i) Using Fig. 1.1, show that the reaction is second order with respect to NOCl. [3] Marker’s Comments: • This part was not well attempted. A large number of students argued that the reaction had to be second order since the graphs did not show zero or first order re action. This is not acceptable as it is not conclusive. Zero, first and second are not the only possible orders of reaction. • Many students tried to calculate rate using 1 time , which is wrong. Rate ≈ 1 time only when the reaction has proceeded to a small extent such that the average rate at that point is approximately equal to initial rate. • Of the students who correctly made use of gradient to calculate the instantaneous rate at various concentrations of NOCl, the common mistakes made were: o Rate was given as a negative value. .. ... .. ... − − − − − − −== − −== − 3 3 1 3 3 1 4 40 1 00rate at 4.00 mol dm 0 5965 mol dm h0 0 5 7 3 10 1 60rate at 2.00 mol dm 0 1500 mol dm h0 0 10 0 Since the rate increases by . . 0 596 0 150 4 times when [NOCl] doubles, the reaction is second order with respect to NOCl.

4 © EJC 9729/02/J2PE/21 For Examiner’s Use o Tangents were not drawn correctly at the stated concentrations. o Points chosen to calculate gradient were too close together. o Points were not accurately read e.g. the initial concentration is not 4.50 mol dm−3. (ii) Given that the rate equation is   2 rate NOCk= l , calculate the rate constant of the reaction, stating its units clearly. [2] Marker’s Comments: • Generally well done. ECF was given based on working in part (i). (iii) A second experiment was conducted under the same conditions, but using half the initial concentration of NOCl. On the axes provided below, sketch the graph that will be obtained using data from the second experiment, relative to that provided for the first experiment. [1] Marker’s Comments: • Poorly attempted. Most students did not realise that the shape of the graph should remain unchanged due to the rate constant, k, remaining unchanged. rate first experiment [NOCl]   ( ) 1 3 10.0373 mol dm h k k k = = = 2 2 rate NOC 0.5965 4.00 −− l

5 © EJC 9729/02/J2PE/21 [Turn Over For Examiner’s Use (d) (i) Define half-life. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ....................... [1] Marker’s Comments: • A number of students lost the mark as they did not state that half-life applied to reactants, and instead merely referred to a “substance” in the definition. (ii) The rate equation for the second -order decomposition of NOC l can also be represented in the following manner:    0 11 NOC NOCt kt−=ll where  NOC tl = concentration of NOCl at time, t,  0NOCl = original concentration of NOCl, and k = rate constant Using this equation, derive an expression for the half-life of the reaction. [1] Marker’s Comments: • Not well -attempted. Many students were not able to see the relationship between  NOC tl and  0NOCl . Others did not understand the meaning of an “expression”, and tried to work out the value of half-life. • Of the student s who managed to derive the expression, some left time as t, without realising that it has been defined to be a generic point in time.           1 2 0 1= NOC t kt kt −= −= 1 2 0 0 0 11 NOC NOC 11 1 NOCNOC2 t k l ll ll Half-life of a reaction is the time taken for the concentration/amount of a reactant to decrease to half its initial value.

6 © EJC 9729/02/J2PE/21 For Examiner’s Use (iii) Hence, state and explain whether the half -life of a second -order reaction will increase or decrease as the reaction proceeds. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ....................... [1] Marker’s Comments: • Students who were not able to derive the expression were unable to provid